This invention relates to a resonant tag circuit design useful in conjunction with an electronic security system for reliably detecting a circuit within a controlled area.
Inventory shrinkage as a result of shoplifting costs U.S. retailers in excess of $10 billion per year. To counteract shoplifting, electronic security systems have been utilized to detect the unauthorized removal of articles from a protected area. These systems utilize radiowaves, microwaves or a magnetic field generated within a confined area through which all articles from the store must pass. A special electronic tag is attached to the article which is sensed by a receiving system to signify the unauthorized removal of the article. If the sensing system does not sense the presence of this special electronic tag within the confined area, then the removal of the article is authorized by virtue of its being paid for and the tag has been either removed from the article at the checkout counter or has been deactivated at the check-out counter. Presently there are two basic types of tags commercially available. One type is a large reusable tag mounted in a plastic housing which is usually fastened to clothing articles; while the other tag is relatively small and disposable and is normally pasted on packages. The large reusable plastic tags are expensive, but can be reused. The small disposable sticker tags can be made at low cost.
A preferred special electronic tag for both the reusable and the disposable applications, utilizes a technology based on tuned circuits that operate in the radio frequency range. To render the tuned circuit functional at the desired frequency, a discrete inductor (L) and discrete capacitor (C) are connected together. The reusable resonant tags use discrete capacitor and inductor components which are connected to form the tuned inductor-capacitor (LC) circuit. In the disposable resonant sticker tag, a discrete inductor and capacitor are formed on a dielectric substrate. Here, the capacitor and inductor are formed by conventional fabrication methods for forming printed circuits including selected use of laminated substrates having an interior dielectric layer laminated on both surfaces with a conductive composition such as aluminum or copper. The conductive layers are printed with an etchant resistant material in the form of the desired circuit and after etching, the remaining conductive material is now in the form of the desired circuit. Such a conventional process is disclosed, for example, in U.S. Pat. No. 3,913,219. Alternatively, the resonant tag circuits can be formed by an additive process whereby an activatable composition is printed upon a dielectric substrate in the form of the desired circuit. The activatable composition then is chemically activated so that, when placed in an electroless bath, it causes reduction of a conductive metal thereon selectively so that metal is not deposited on those areas which are not chemically activated. The electrolesscoated pattern then can be further coated with metal by conventional plating techniques to form a resonant tag circuit. Alternately, the resonant tag circuits can be formed by an additive process whereby the film is chemically treated to render it platable and a plating mask is used to prevent plating in noncircuit areas. Alternatively, the resonant tag circuits can be formed by stamping, dye cutting, precision fine blanking or other form of stamping the circuit out of thin metal sheets and laminating the two sides to the circuit on opposite sides of a film.
Prior to the present invention, the resonant tag circuits were formed with discrete inductor and capacitor components. Such tags are shown, for example, in U.S. Pat. Nos. 3,967,161; 4,021,705; 3,913,219; 4,369,557; 3,810,147 and 3,863,244. These prior art tags, by virtue of their use of an inductor and a capacitor as separate elements introduce inherent limitations in the disposable resonant sticker tag produced therewith.
In these resonant tag circuits, it is desirable to produce tags that operate at specific frequencies. Specific frequencies can be obtained by varying L and/or C based on the equation: ##EQU1## In general, it is also desirable to have a sharp resonance curve where there is a large change in impedance as a function of frequency over a narrow frequency range in order to provide the desired selectivity to discriminate between tuned circuits and environmental interferences.
The sharpness of the resonance curve is usually determined by a quality factor called "Q" which can be defined as the ratio of the reactance of either the coil or the capacitor at resonant frequency to the total resistance. It is also a measure of the reactive power stored in the tuned circuit to the actual power dissipated in the resistance. The higher the "Q", the greater the amount of energy stored in the circuit compared with the energy lost in the resistance during each cycle.
Therefore, it is generally desirable to have a resonant tag circuit with a high "Q" factor.
Mathematically: ##EQU2## Where X.sub.L =Inductive reactance
X.sub.C =Capacitive reactance PA0 L=Inductance PA0 C=Capacitance PA0 f=Frequency PA0 R=Resistance. PA0 (b) Increasing the inductance (L)
Combining equations 2 and 3: ##EQU3## which indicates "Q" can be improved by: (a) Lowering the resistance (R)
(c) Reducing the capacitance (C).
The "Q" factor is also related to the power stored in the resonant tag circuit which means, the dielectric loss of the substrate should be minimized to improve the "Q" factor. This dielectric loss is normally referred to as the dielectric dissipation factor of the capacitor.
Assuming the dielectric dissipation factor for a particular class of resonant tag circuits is constant, then lowering the series resistance, increasing the inductance and/or lowering the capacitance, are three possible variables that can be changed to improving the "Q" factor for a resonant tag circuit tuned at a specific frequency.
The most obvious approach for improving "Q" is to reduce the resistance (R). Difficulty in improving "Q" is increased when "Q" is to be improved by adjusting the L/C ratio because when L is increased, C must be reduced to maintain the desired frequency; and in most cases, the resistance (R) increases as L is increased because an increase in inductance is usually associated with a longer inductor.
In many applications, it is desirable to have a resonant tag that is relatively small, inexpensive to made and functions as an antenna allowing it to be sensed by detecting equipment. Adding these three additional objectives to making a high "Q" factor resonant tag circuit that functions at a specific frequency further complicates the circuit design when using a discrete inductor in combination with a discrete capacitor.
Presently available resonant sticker tag circuits are produced by an etching process. U.S. Pat. No. 3,863,244, issued Jan. 28, 1975, and U.S. Pat. No. 3,967,161, issued June 29, 1976, disclose resonant tag circuits which are fabricated by etched circuit techniques. The tag circuit comprises an insulatve substrate having one portion of the circuit formed on one side of the substrate and another portion of the circuit formed on the opposite side of the substrate. Electrical connection is made between the portions of the circuit on opposite sides of the substrate by means of a conductive pin or eyelet extending through the substrate, or by means of a spot weld joining confronting circuit areas. U.S. Pat. No. 4,021,705, issued May 3, 1977, discloses a similar type of resonant tag circuit.
U.S. Pat. No. 3,913,219, issued Oct. 21, 1975, discloses a fabrication process for planar resonant tag circuits. in which both sides of a web of insulative material are provided with a conductive material to serve as conductive surfaces from which circuit patterns are formed by etched circuit techniques. Electrical connection is established between the two conductive patterns on opposing faces to the web by welding confronting conductive surfaces, such as by ultrasonic welding or cold-welding with the aid of a tool having a chisel-like tip.
These resonant tag circuits require a relatively long and thin inductor line with many turns and a large capacitor plate (FIGS. 1-4) to properly tune the resonant tag circuit, but this fine circuit line with many turns is difficult to produce by etching techniques without having shorts or delamination of the conductive material. This same fine circuit line and large capacitor plate is equally, if not more difficult, to make by an additive process where electroplating may be required to reduce the resistance. The fine circuit line cannot carry a substantial amount of current, thereby requiring an extremely long time to deposit conductive metal onto this fine line. When plating is finished, the portion of the line closest to the electroplating connection would be plated to a much greater thickness than other portions of the line with the excessive amounts of conductive material being an undesirable increase in the cost of the overall circuit. This same fine circuit line and capacitor plate would be equally difficult to produce by stamping and laminating techniques because the inductor coil is long and thin and a relatively large capacitor is located along or at the end of this thin coil making it difficult to handle after stamping and equally difficult to place on an insulative substrate and properly align prior to lamination.
Accordingly, it would be highly desirable to provide a resonant tag circuit which is resonant at a desired frequency, has a high "Q" factor, is relatively small in size, can be detected by existing detection equipment and can be made economically and quickly by etching, additive plating and stamping/laminating techniques.